Structural foam as a plastic product has an integral skin and cellular core. The combination of the skin, which is normally solid, i.e., without voids or cells, and the cellular core produces a relatively high strength to weight ratio manifested as high stiffness. Unfortunately, because of the presence of the cellular core, the increase in stiffness of the structural foam is accompanied by a decrease in toughness. When the structural foam is in the form of a pallet, it typically has sufficient rigidity for holding and transporting a load, but the pallet tends to crack and break upon the rough handling typical of pallet usage.
U.S. Pat. No. 3,268,636 discloses the basic process for making structural foam articles by melting thermoplastic resin and forcing it into a mold in the presence of a blowing agent to mold the article. Upon entry of the molten resin into the mold, the blowing agent foams the resin to create the cellular core of the molded article. The mold is kept at a temperature below the melting temperature of the resin so that the resultant rapid solidification of the resin coming into contact with the surface of the mold keeps this resin relatively solid, i.e., non-cellular, to form the skin of the molded article. The resin which does not come into contact with the mold surface remains sufficiently fluid in the mold to permit its foaming by the blowing agent. Example 1 of U.S. Pat. No. 3,268,636 discloses a mold temperature of 320.degree. F. (160.degree. C.) when high density polyethylene was used as the resin to produce a bowling pin having a dense outer shell having a polished-like surface and a cellular core. Example 2 discloses the resin to be a 50:50 blend of polyethylene with polypropylene and a mold temperature of 70.degree. F. (21.degree. C.) to produce a structural foam bowling pin having a less desirable grainy texture similar to wood.
The resins used in this patent were semicrystalline resins which have melt characteristics (high melt viscosity and high melt strength) which make them easy to foam with the use of the blowing agent. Indeed, the most popular structural foam pallet of thermoplastic resin used today is that wherein the resin is high density polyethylene.
Polyester resins are thermoplastic resins that typically can be crystallized from the molten state. These resins, however, have found relatively small usage in the structural foam application. Polybutylene terephthalate (PBT) available as Valox.RTM. resins with 10% to 30% glass reinforcement is supplied by the General Electric Company for structural foam application. This is a relatively expensive polyester resin and is well known to crystallize rapidly upon cooling down from the molten state. The publication "Structural Foam" published by the Society of the Plastics Industry (believed published in the 1970's) discloses on page 6 the Charpy impact (un-notched) and test result of only a 5.3 ft. lbs/in (283 J/m) for structural foam articles made from "Thermoplastic Polyester" which is believed to be PBT.
The less expensive polyethylene terephthalate (PET) has not found utility in structural foam articles, such as pallets. PET differs from PBT by crystallizing less rapidly from the molten state, enabling even the amorphous state to be achieved if the cooling is rapid enough to a temperature below the glass transition temperature (Tg) of the PET which is about 75.degree. C. When an article of PET is exposed to temperatures above 75.degree. C., however, it tends to lose its toughness properties, because above this temperature, the PET crystallizes. This crystallization occurring upon reheating is rather coarse-grained crystallization, which has led the PET art to use crystallization promoters to induce fine-grain crystallization state to give better toughness properties. These crystallization promoters promote the formation of fine grain crystallization at the expense of amorphous PET. The unique attributes of PET, as well as the attributes of low melt viscosity and low melt strength, of this popular and widely used resin have led to the utility of PET in fields other than foam.